Re: [PATCH v3 04/10] rust: list: add struct with prev/next pointers

[Alice Ryhl]

On Thu, Aug 1, 2024 at 3:46 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On 01.08.24 14:51, Alice Ryhl wrote:
> > On Thu, Aug 1, 2024 at 12:45 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> >>
> >> On 01.08.24 11:42, Alice Ryhl wrote:
> >>> On Wed, Jul 31, 2024 at 8:41 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> >>>>
> >>>> On 23.07.24 10:22, Alice Ryhl wrote:
> >>>>> +/// The prev/next pointers for an item in a linked list.
> >>>>> +///
> >>>>> +/// # Invariants
> >>>>> +///
> >>>>> +/// The fields are null if and only if this item is not in a list.
> >>>>> +#[repr(transparent)]
> >>>>> +pub struct ListLinks<const ID: u64 = 0> {
> >>>>> +    #[allow(dead_code)]
> >>>>> +    inner: Opaque<ListLinksFields>,
> >>>>
> >>>> Do you really need `Opaque`? Or would `UnsafeCell` be enough? (If it is
> >>>> enough and you change this, be aware that `Opaque` is `!Unpin`, so if
> >>>> you intend for `ListLinks` to also be `!Unpin`, then you need a
> >>>> `PhantomPinned`)
> >>>
> >>> I need the `!Unpin` part for aliasing.
> >>
> >> Oh good point, do you mind adding a comment for that?
> >>
> >>>>> +}
> >>>>> +
> >>>>> +// SAFETY: The next/prev fields of a ListLinks can be moved across thread boundaries.
> >>>>
> >>>> Why? This is not a justification.
> >>>
> >>> What would you say?
> >>
> >> While trying to come up with a safety comment I thought about the
> >> following: this impl does not depend on the type that is behind the
> >> pointer (ie the type containing the `ListLinks`). Thus this `ListLinks`
> >> will always implement `Send` even if the pointed-to value does not.
> >> What we could do (and what definitely would be correct) is this:
> >> `List` can only be used with `Send` types, then we could implement
> >> `Send` for `ListLinks`. But I haven't actually come up with a problem,
> >> so there might a more permissive solution.
> >> Do you have a use-case where you need `!Send` types in a list?
> >>
> >> Here is a part of my reasoning: If the pointed-to value is `!Send`, then
> >> the `List` item type must also be `!Send`. Thus all list operations take
> >> place on the same thread (since the `List` will be `!Send`). Therefore
> >> nobody can access the `prev`/`next` pointers from another thread.
> >>
> >> But this does not justify that `ListLinks` can be made `Send`. (although
> >> there isn't actually a problem)
>
> I think I confused myself. The paragraph above actually explains why we
> are allowed to make `ListLinks: Send`. What do you think of the
> following comment:
>
> // SAFETY: The only way to access/modify the pointers inside of `ListLinks<ID>` is via holding the
> // associated `ListArc<T, ID>`. Since that type correctly implements `Send`, it is impossible to
> // move this an instance of this type to a different thread if the pointees are `!Send`.

I will use that, thanks.

Alice